At first you have to clone a project from a GitHub Repo
>> git clone git://github.com/mhoofman/wordpress-heroku.git your_app_name
Then you can create your app
>> cd your_app_name
>> heroku create your_app_name
Now you should create a database
>> heroku addons:add heroku-postgresql:dev
In the root folder of your app exists a file called wp-config-sample.php rename it to wp-config.php
In this file you will find a few lines like define('AUTH_KEY',...) and so on.
Replace this lines by some random generated lines from here: https://api.wordpress.org/secret-key/1.1/salt/